The instant invention relates to liquid tertiary amine accelerators which are used to provide latency for anhydride curing agents for epoxy resins.
Certain anhydrides are known for use as curing agents for epoxy resins. The commercially known anhydrides possess the advantage of producing only mild skin irritation compared to amine curing agents and generally provide acceptable viscosity and pot life. Epoxy resins cured with anhydrides generally exhibit high temperature stability, good radiation stability as well as useful physical and electrical properties above their deflection temperature (DT).
The reaction of anhydrides with epoxy resins is dependent upon a number of factors including, for example, the gel time and temperature, post-cure and post cure temperature, presence or absence of accelerators, type of accelerator, amount of hydroxyl group in the resin, ratio of anhydride to epoxy and the amount of free acid in the system. Anhydrides will typically not react with epoxy groups in the absence of an accelerator.
Typical commercial epoxy-resin/anhydride formulations use anhydride accelerators. These are acidic or basic compounds. Acids favor etherification while bases favor esterification. The optimum anhydride/epoxy ratio (A/E) and the cured properties of the resin are determined by the accelerator used. Tertiary amines are conventionally used as anhydride accelerators. These conventional amines are described in Three Bond Technical News vol. 32, Dec. 20, 1990. Conventional amines include benzlydimethylamine (BDMA) and tris(dimethylaminomethyl)phenol, triethylene diamine (TEDA), N,N-dimethylpiperazine and 2-(dimethylaminomethyl)phenol. Imidazoles such as 2-methylimidazole, 2-Et-4-methylimidazole,1-cyanoethyl-2-undecylimidazolium trimellitate and the epoxy-imidazole adduct (2-methylimidazole/Epon 828).
U.S. Pat. No. 3,839,281 discloses using N-hydroxyethyl piperidines and piperazyl compounds as accelerators for epoxy resins systems cured with anhydrides and dicyandiamide (DICY). U.S. Pat. No. 5,650,477 discloses quaternary ammonium salts bearing an ether linkage with a nitrile group were used as catalysts for anhydride cured expoxy resins under microwave irradiation. Solid metal acetylacetonates are described as latent curing agents in J. Appl. Poly. Sci, 26, 1981, 979 by J. Smith. These solid metal acetylacetonates have the disadvantage of not being able to be dispersed adequately to effect efficient curing of epoxy resins by anhydrides.
The previously disclosed patents, patent applications and documents are hereby incorporated by reference.
There is a need in this art for anhydride accelerators with improved latency in order to minimize the waste of the pre-mixed anhydride system thereby providing a significant saving in raw material cost as well as cured epoxy systems having desirable physical properties.